[Cython] prange CEP updated

[mark florisson]

On 4 May 2011 14:10, Dag Sverre Seljebotn <d.s.seljebotn at astro.uio.no> wrote:
> On 05/04/2011 01:59 PM, mark florisson wrote:
>>
>> On 4 May 2011 13:54, Dag Sverre Seljebotn<d.s.seljebotn at astro.uio.no>
>>  wrote:
>>>
>>> On 05/04/2011 01:48 PM, mark florisson wrote:
>>>>
>>>> On 4 May 2011 13:47, mark florisson<markflorisson88 at gmail.com>    wrote:
>>>>>
>>>>> On 4 May 2011 13:45, Dag Sverre Seljebotn<d.s.seljebotn at astro.uio.no>
>>>>>  wrote:
>>>
>>>>>> Look.
>>>>>>
>>>>>> i = 42
>>>>>> for i in prange(n):
>>>>>>    f(i)
>>>>>> print i # want 42 whenever n == 0
>>>>>>
>>>>>> Now, translate this to:
>>>>>>
>>>>>> i = 42;
>>>>>> #pragma omp parallel for firstprivate(i) lastprivate(i)
>>>>>> for (temp = 0; ...; ...) {
>>>>>>    i = ...
>>>>>> }
>>>>>> #pragma omp parallel end
>>>>>> /* At this point, i == 42 if n == 0 */
>>>>>>
>>>>>> Am I missing something?
>>>>>
>>>>> Yes, 'i' may be uninitialized with nsteps>    0 (this should be valid
>>>>> code). So if nsteps>    0, we need to initialize 'i' to something to
>>>>> get
>>>>> correct behaviour with firstprivate.
>>>
>>> This I don't see. I think I need to be spoon-fed on this one.
>>
>> So assume this code
>>
>> cdef int i
>>
>> for i in prange(10): ...
>>
>> Now if we transform this without the guard we get
>>
>> int i;
>>
>> #pragma omp parallel for firstprivate(i) lastprivate(i)
>> for (...) { ...}
>>
>> This is invalid C code, but valid Cython code. So we need to
>> initialize 'i', but then we get our "leave it unaffected for 0
>> iterations" paradox. So we need a guard.
>
> You mean C code won't compile if i is firstprivate and not initialized?
> (Sorry, I'm not aware of such things.)

It will compile and warn, but it is technically invalid, as you're
reading an uninitialized variable, which has undefined behavior. If
e.g. the variable contains a trap representation on a certain
architecture, it might halt the program (I'm not sure which
architecture that would be, but I believe they exist).

> My first instinct is to initialize i to 0xbadabada. After all, its value is
> not specified -- we're not violating any Cython specs by initializing it to
> garbage ourselves.

The problem is that we don't know whether the user has initialized the
variable. So if we want firstprivate to suppress warnings, we should
assume that the user hasn't and do it ourselves.

> OTOH, I see that your approach with an if-test is more Valgrind-friendly, so
> I'm OK with that.
>
> Would it work to do
>
> if (nsteps > 0) {
>    #pragma omp parallel
>    i = 0;
>    #pragma omp for lastprivate(i)
>    for (temp = 0; ...) ...
>    ...
> }

I'm assuming you mean #pragma omp parallel private(i), otherwise you
have a race (I'm not sure how much that matters for assignment). In
any case, with the private() clause 'i' would be uninitialized
afterwards. In either case it won't do anything useful.

> instead, to get rid of the warning without using a firstprivate? Not sure if
> there's an efficiency difference here, I suppose a good C compiler could
> compile them to the same thing.
>
> Dag Sverre
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